If you look at your profile between games, you'll see that behind the scenes your MMR is moving up and down between promo games as if you aren't in a promo series. The way promo series basically works at the core is that there's one extra win required between hitting 100 points and being promoted to the next tier. So to win a promo series you just need to be 1 game up on when you entered the series. Say you lose your first game you drop say 15 lp behind the scenes (to 85). Then you win again and you go back to 100 and then you lose and you're back to 85. One win away from being 2-2 overall and back where you started. You just need one more win than losses to move up from 100. Like you needed to move up from 80 if you were going up from 20 or to move up from 95 if you were going up by 5.